The present invention relates to semiconductor devices and wire bonding methods.
Assembling steps of a semiconductor device such as an IC includes a wire bonding step for connecting between a chip and a lead frame of a semiconductor using a wire. In a commonly employed method for the wire bonding step, a capillary through which a wire is threaded is used, a discharge from a flame off electrode forms a ball at a tip end of the wire protruding from the capillary, the capillary is moved above a pad of a semiconductor chip to perform primary bonding, and then the capillary is moved above a lead of the lead frame to perform secondary bonding, thereby connecting between the semiconductor chip and the lead frame using the wire (for example, see Japanese Patent No. 3570551).
In this case, a gold wire is typically used as the wire. Although gold exhibits excellent bondability with the same gold material, the bondability of gold with a material such as copper, nickel, or flash gold plating is low. In a case in which the pad or the lead is made of a material whose bondability with gold wire is low, such as copper, nickel, or flash gold plating, for example, the gold wire cannot be bonded directly on the bad or the lead. For this reason, in a commonly employed method, a bump is formed on the pad or the lead by ball bonding using a gold wire, and then the gold wire is bonded on the bump.
According to this method, the bondability of the gold wire increases because the gold wire is bonded on the bump that is made of the same gold material and the bondability between the gold wire and the pad or the lead that is made of a material having low bondability with gold can be improved. However, problems have been noted that, when there is not provided a sufficient size of a slope and a flat surface on an upper surface of the bump for the bonding by the gold wire, a bonding defect can occur due to a lack of the bonding area, i.e. the gold wire deforms downward after bonding the bump with the gold wire and is brought into contact with the lead frame or a semiconductor chip 2.
Therefore, in order to provide a sufficient size of a slope and a flat surface on an upper surface of the bump for the bonding by the gold wire, Japanese Patent No. 3570551 and Japanese Patent Application Unexamined Publication No. 2004-247672 propose a method of forming a slope wedge by pushing the capillary after ball bonding to bond a gold wire to the slope wedge on the bump.
According to this method, as shown in (a) in FIG. 15, for example, a bump 21 comprised of a first bump layer 21a and a second bump layer 21b is formed on a pad 3 of the semiconductor chip 2 so that a direction of a slope surface of a slope wedge 22 is opposite from a lead 4, and a ball 5 is formed at a tip end of a wire 12 by a discharge spark to the wire 12. Then, there is a wire bonding method in which primary bonding is performed in which the ball 5 is pressed on the lead 4 to form a pressure-bonded ball 6 as shown in (b) in FIG. 15. After secondary bonding is performed in which a capillary 16 is moved from the lead 4 toward the pad 3 to loop the wire 12 and the wire 12 is pressed onto the slope wedge 22 of the bump 21 as shown in (c) in FIG. 15, the wire 12 is cut. Further, there is a wire bonding method in which, as shown in FIG. 16, the primary bonding is performed to the pad 3 of the semiconductor chip 2 by forming the bump 21 having the slope wedge 22 whose direction of the slope surface is opposite from the semiconductor chip 2 on the lead 4 to form the pressure-bonded ball 6, and then, the secondary bonding is performed to the slope wedge 22 of the bump 21 by looping the wire 12 toward the lead 4. According to these methods, the wire 12 is bonded along the slope wedge 22 of the bump 21. This increases the bonding area of the slope wedge 22, and improves the bonding intensity. In addition, because the wire 12 is supported by the slope wedge 22, it is possible to prevent the wire 12 from being brought into contact with the semiconductor chip or the lead frame 15.
On the other hand, in cutting the wire in the wire bonding step, a problem has been noted that a bend is caused at the tip end of the wire. This results in a problem that the connected wire after the bonding is bent in an S shape and a defect that adjacent wires are brought into contact with each other can occur. To address this problem, methods have been proposed, for example, such that positions of the capillary and a clamper are shifted in a traverse direction before the wire is pulled up to be cut, and then, a thin wire bent portion is pulled directly upward and cut (for example, see Japanese Patent No. 2723277), and such that, after a tail wire is lead out by opening the clamper, the capillary is vibrated at a characteristic frequency of the tail wire to have the tail wire vibrate sympathetically and the wire is cut at the thin wire bent portion, thereby preventing the wire bending from occurring when cutting the wire (for example, see Japanese Patent No. 2969953).
However, as shown in FIG. 17, when the bonding area between the wire 12 and the bump 21 is increased by bonding the wire 12 onto the slope wedge 22 on the bump 21, the shape of a wire bent portion 20 between the wire 12 and the bump 21 does not become thin in some cases. In such a case, in the conventional art as described in Japanese Patent No. 2723277, a large tensile force is applied on the wire 12 and the wire 12 is cut after being extended by the thick wire bent portion 20, when the clamper 17 holds the wire 12 to pull the wire up. Accordingly, the extended wire 12 is bent upward due to reaction force in the cutting and forms the S shape. The bend of the wire 12 causes problems such as a defect in the ball formation due to a discharge when bonding the pad 3, and a defect that the wire 12 is bent to form the S shape as shown in FIG. 18, and the adjacent wires 12 are brought into contact with each other because of the bonding by the bent wire remaining in the capillary 16 and between the capillary 16 and the clamper 17.
Further, the conventional art as described in Japanese Patent No. 2969953 adversely requires an application of ultrasonic vibration for an extended period of time in order to cut the thick wire bent portion 20, and therefore cannot be applied to a high speed bonding apparatus using a short tail wire, since the technique of No. 2969953 cannot use a short tail wire because resonance of a tail wire 18 of the wire 12 that is lead between the capillary 16 and a bonding point is created.
As described above, when connecting between the pad 3 of the semiconductor chip 2 or the lead 4 of the lead frame 15 that are made of a metal material having low bondability with the wire using the wire 12, formation of the bump 21 having a shape providing excellent bondability in order to improve the bondability and cutting property of the wire 12 after bonding to the bump 21 are conflicting. In the conventional art, a problem has been noted that an improvement of the bondability lowers the cutting property of the wire 12, and thus results in an occurrence of the bend of the wire 12.